This application is related to Japanese application No. HEI10(1998)-304979 filed on Oct. 10, 1998, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a process for preparing a positive electrode active material for a nonaqueous secondary battery and a nonaqueous secondary battery using the active material. More particularly, it relate to a process for producing LiNi1xe2x88x92XMXO2, wherein 0xe2x89xa6X less than 0.5 and M is at least one element selected from transition metals and 3B-, 4B- and 5B- group elements, and a nonaqueous secondary battery using LiNi1xe2x88x92XMXO2 as a positive electrode active material.
2. Description of Related Arts
The following are reported as conventionally typical processes for preparing lithium nickel oxide (LiNiO2).
In an example of the process for preparing LiNiO2 shown in the Journal of the American Chemical Society (J.Am.Chem.Soc.), 76,1499(1954), solid anhydrous lithium hydroxide and metallic nickel are mixed and calcined in an atmosphere of air.
In an example of the process for preparing LiNiO2 disclosed by Japanese Unexamined Patent Publication No. HEI 2(1990)-40861, solid lithium hydride monohydrate and solid nickel oxide are mixed. The resulting mixture is first calcined at 600xc2x0 C. in an atmosphere of air, and then the calcined mixture is pulverized and calcined again at 600xc2x0 C. to 800xc2x0 C.
In an example of the process for preparing LiNiO2 disclosed by Japanese Unexamined Patent Publication No. HEI 5(1993)-205741, solid lithium peroxide and nickel oxide are mixed and calcined at 750xc2x0 C. or lower, followed by quenching.
In an example of the process for preparing LiNiO2 disclosed by Japanese Unexamined Patent Publication No. HEI 5(1993)-251079, solid lithium nitrate is mixed with at least one of solid nickel hydroxide and nickel oxyhydroxide and the resulting mixture is calcined at 500xc2x0 C. to 1,000xc2x0 C.
In an example of the process for preparing LiNiO2 disclosed by Japanese Unexamined Patent Publication No. HEI 6(1994)-203834, nickel acetate and lithium acetate are dissolved in ethylene glycol with heating and solidified by heating further. The resulting mixture is thermally treated at 400xc2x0 C. in air, pulverized and calcined at 700xc2x0 C. in a stream of oxygen. The calcined mixture is calcined again at 800xc2x0 C. in a stream of oxygen.
In an example of the process for preparing LiNiO2 shown in the Chemistry Express, 6,161(1991), 4.5M lithium hydroxide aqueous solution and 1.0M nickel nitrate aqueous solution are mixed equimolarly at 60xc2x0 C. The resulting solution is evaporated to dryness by stirring under reduced pressure. The resulting solid is pulverized, and calcined preliminarily at 300xc2x0 C. and then at 800xc2x0 C.
In an example of the process for preparing a composite oxide of lithium and nickel disclosed by Japanese Unexamined Patent Publication No. HEI 6(1994)-44970, to a saturated aqueous solution of at least one nickel salt selected from a halogenated nickel, nickel sulfate, nickel phosphate, nickel acetate and nickel oxalate, added is a saturated aqueous solution of at least one lithium salt selected from lithium hydroxide, lithium carbonate and lithium hydrogen carbonate in an equimolar amount with the nickel salt. The resulting mixture is evaporated to dryness with stirring in air or under reduced pressure. The resulting cake-like solid is calcined at 600xc2x0 C. to 800xc2x0 C.
In an example of the process for preparing a composite oxide of lithium and nickel disclosed by Japanese Unexamined Patent Publication No. HEI 6(1994)-44971, to powders of at least one nickel compound slightly soluble or insoluble in water selected from nickel oxide, nickel oxyhydroxide, nickel hydroxide and nickel carbonate, added is a saturated aqueous solution of at least one lithium salt selected from a halogenated lithium, lithium nitrate, lithium sulfate, lithium phosphate, lithium borate, lithium acetate and lithium oxalate. The resulting mixture is sufficiently kneaded with stirring. The mixture is evaporated to dryness with stirring in air or under reduced pressure. The resulting cake-like solid is calcined at 600xc2x0 C. to 800xc2x0 C.
In an example of the process for preparing LiNiO2 disclosed by Japanese Unexamined Patent Publication No. HEI 6(1994)-96769, a lithium source and a nickel source are mixed so that the ratio of the molar amount of lithium in the lithium source to that of nickel in the nickel source is 1:1. At this time, a small amount of water is added as a dispersant. The resulting mixture is dried and calcined at 650xc2x0 C. to 800xc2x0 C.
In an example of the process for preparing LiNiO2 disclosed by Japanese Unexamined Patent Publication No. HEI 9(1997)-156931, powders of nickel oxide and a lithium compound of either lithium hydroxide or lithium nitrate are weighed so that the molar ratio of lithium to nickel is 1:1. The nickel oxide powders are allowed to soak the lithium compound melted at a temperature of melting temperature thereof or higher to 500xc2x0 C. or lower. The resulting mixture is calcined in the presence of oxygen or an oxygen-rich gas.
Of the above-described processes for producing positive electrode active materials for nonaqueous secondary batteries, in the processes of mixing a lithium compound and a nickel compound in solid states, there is a problem in that the mixed state of both the compounds is not uniform prior to calcination.
In the processes of mixing a lithium compound and a nickel compound in aqueous solutions, the solutes do not precipitate simultaneously while the mixed solution is evaporated to dryness, because the solutes have different solubilities. Therefore, the lithium compound and the nickel compound are not uniformly mixed in the resulting solid.
As regards the process of dissolving nickel acetate and lithium acetate in ethylene glycol with heating, evaporating the resulting solution to dryness with further heating and calcining the resulting solid twice, it is not preferable since it requires a long calcination time and involves complicated production steps. Furthermore, the mixed state of the lithium compound and the nickel compound is still insufficient in the solid obtained by this process before calcination.
In the process of dispersing in a powdery nickel compound slightly soluble or insoluble in water in an aqueous solution of a lithium salt and the process of dispersing and kneading them with stirring a lithium compound and a nickel compound in water, the resulting matter is not a uniform mixture of the lithium compound and the nickel compound because the solutes do not precipitate at the same time while water as a dispersant is being removed.
Also in the process of allowing a melted lithium salt to penetrate into a powdery-nickel oxide, the mixed state of nickel oxide and the lithium salt is somewhat improved, but is still insufficient.
In nonaqueous secondary batteries using matters obtained by calcining insufficiently uniform mixtures as positive electrode active materials, their discharge capacity declines remarkably with increase of the number of charge-discharge cycles, and electrodes thereof deteriorate quickly.
In order to solve the above-mentioned problems, according to the process for producing LiNiO2 disclosed by Japanese Unexamined Patent Publication No. HEI 10(1998)-106564, a water-soluble lithium compound and a water-soluble nickel compound are dissolved in water to prepare an aqueous solution. Subsequently, oxalic acid is added to the resulting aqueous solution to co-precipitate a lithium salt and a nickel salt. The resulting precipitate is dried and then calcined.
By this co-precipitation, the lithium salt and the nickel salt in the precipitate are in a uniformly mixed state. Accordingly, nonaqueous secondary batteries using LiNiO2 obtained by calcining the precipitate as a positive electrode active material are improved in charge-discharge cycle characteristics.
In the above-described process for producing LiNiO2 disclosed by Japanese Unexamined Patent Publication No. HEI 10(1998)-106564, since lithium and nickel are allowed to precipitate in a composite oxalate, it is possible to obtain a precursor in which the lithium compound and the nickel compound are uniformly mixed. However, because lithium oxalate is relatively readily dissolved in water, the molar ratio of lithium to nickel (lithium/nickel) in the precursor varies every time when the precursor is prepared. Therefore reproducibility is not sufficient.
Furthermore, in the step of preparing the aqueous solution of the lithium compound and the nickel compound before oxalic acid is added, the lithium compound as a starting material must be used in an amount about several times to about ten times larger than the nickel compound in terms of the molar ratio of lithium to nickel.
For this reason, although nonaqueous secondary batteries using, as a positive electrode active material, LiNiO2 obtained by calcining the precursor produced by the co-precipitation are improved in an initial discharge capacity and have good charge-discharge cycle characteristics, there are problems that the process fails to provide active materials stable in quality, costs much for starting materials and involves a great waste of resources.
Under these circumstances, an object of the present invention is to provide a process for producing a positive electrode active material, LiNi1xe2x88x92XMXO2, for nonaqueous secondary battery which material can exhibit stable charge-discharge characteristics, at a reduced cost without wastage of resources and also to provide a lithium ion secondary battery which can exhibit good charge-discharge characteristics.
According to the process for producing a positive electrode active material for a nonaqueous secondary batteries of the present invention, compounds capable of providing lithium ions, nickel ions and optionally ions of at least one element selected from transition metals and 3B-, 4B- and 5B-group elements are firstly dissolved in a buffer solution containing at least lithium ions to prepare a starting aqueous solution. A dicarboxylic acid is added to the starting aqueous solution to form a precipitate containing salts of the above-mentioned ions with the dicarboxylic acid, and the precipitate is separated. The separated precipitate is calcined to produce LiNi1xe2x88x92xMxO2, wherein 0xe2x89xa6X less than 0.5 and M is at least one element selected from transition metals and 3B-, 4B- and 5B-group elements. Residual liquid after the separation of the precipitate can be re-used for preparing the above-described starting aqueous solution.
With this constitution, since the residual liquid is recycled in the production process of the positive electrode active material, it is possible to reduce costs for raw materials and to use resources effectively.
According to the present invention, there is also provided a nonaqueous secondary battery comprising a positive electrode containing a positive electrode active material produced as described above, a negative electrode and an ion conductor.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.